An Optical Parametric Oscillator (OPO) is a device employing a non-linear crystal which when pumped by a laser can generate coherent light whose wavelength can be varied continuously over a wide range. In the OPO the non-linear crystal (such as BaB.sub.2 O.sub.4, LiB.sub.3 O.sub.5, LiNbO.sub.3, KTiOPO.sub.4 and others) is placed in an optical resonator. When a laser beam, at a wavelength .lambda..sub.p, is directed to propagate through the crystal, a pair of variable wavelength beams are produced. Energy is conserved so: ##EQU1##
By convention, the laser wavelength, .lambda..sub.p, is dubbed as the pump beam, the wavelength .lambda..sub.s is referred to as the signal wavelength and .lambda..sub.i is the idler wavelength, where the signal has a shorter wavelength than the idler (.lambda..sub.s &lt;.lambda..sub.i). The wavelengths of these beams can be tuned over a wide spectral range by varying the orientation of the crystal with respect to the laser beam, by changing the crystal's temperature, or by applying a variable voltage across the crystal. Various tuning ranges can be achieved by properly selecting the laser, the non-linear crystal, and the optical components.
Under typical operating conditions, the initial generation of the parametric beams (the idler and the signal) is inefficient and only a small fraction of the pump beam is converted after a single pass through the crystal. The efficiency is significantly improved by oscillating one or both of the parametric beams such that it (or they) are amplified in successive passes through the crystal(s). The oscillator is comprised of optical elements designed to provide the required feedback for efficient conversion. The principles of OPO are well known and described in texts on lasers and non-linear optics (for example, A. Yariv, Quantum Electronics, 3.sup.rd edition, p. 411. John Wiley & Sons, New York).
The performance of an OPO is characterized by various parameters such as wavelength tuning range, conversion efficiency, spectral line width and beam quality. Other attributes such as low cost, long-term stability, robust design and ease of operation are important in making the OPO a practical device. For a given pump beam, the design of the oscillator dictates the performance of the OPO.
Various constrain affect the design, and therefore the performance of presently available OPO devices. Most importantly, the desire to simultaneously achieve high conversion efficiency while enabling wide wavelength tunability imposes conflicting design demands that lead to tradeoff and result in compromised performance. The tuning range defines the versatility of the OPO and the conversion efficiency is strongly related to the practicality of the device. The most attractive feature of the parametric process is the potential for generating a widely tunable, high quality laser beam. The tuning of an OPO is in general dictated by the characteristics of the selected non-linear crystal (e.g., non-linear coefficient, birefringency, transmission and physical size) and the design of the coating of the associated cavity optics.
Enabling wide wavelength tuning requires, in prior art devices, that the oscillator incorporates optics with multi-layer dielectric coatings that have to satisfy complex requirements for reflectivity and transmission at the pump, signal, and idler wavelengths. Coatings that can satisfy the requirements for a wide tuning range are difficult and sometime impossible to fabricate. Each tuning range requires custom coatings and therefore these optics are not readily available for many ranges and they tend to be expensive. Moreover, the multi-layer coatings have relatively low optical damage threshold, which limits the pump intensity and adversely affect the conversion efficiency of the OPO.
Conversion efficiency is defined as the ratio of the output energy of the OPO beam (signal or idler) to the energy of the laser (pump) beam. The conversion efficiency of an OPO is associated with complex and inter-related design parameters, most importantly: oscillator design, pumping configuration and cavity length.
Oscillation
The oscillator can be designed to oscillate either one of the parametric beams (Single Resonance Oscillator, SRO) or both of them (Double Resonance Oscillator, DRO). The DRO design provides lower threshold and higher conversion efficiency whereas the SRO design offer advantages in special applications such as narrow-line operation. Although the DRO is usually the preferred configuration for high power OPO's, it is difficult to manufacture coatings for DRO's that will operate over a wide spectral range.
Pumping Configuration
The optical parametric process requires that the signal and idler beams propagate approximately along the same direction as the pump beam. In simple OPO designs the pump beam traverses the crystal only in one direction whereas the parametric beam oscillate back and forth. In this case the interaction between the pump and the parametric beams is limited to only half the time the parametric photons are oscillating inside the cavity, resulting in low conversion efficiency. To maximize efficiency, OPO's have been designed such that the pump beam as well as the parametric beams will transverse the crystal more than once. This can be achieved in a ring oscillator design (such as that described in U.S. Pat. No. 5,276,548) or by retro reflecting the pump beam after it has passed through the crystal to create what is known as a double pump oscillator (DPO). (See, for example, Brosnan, Optical Parametric Oscillator Threshold and Linewidth Studies. IEEE JQE Vol.15, No 6, June 1979; Guyer, U.S. Pat. No. 5,079,445; Nabor, U.S. Pat. No. 5,781,571 and Harlamoff, U.S. Pat. No. 5,406,409).
In prior art linear cavity designs, additional optics are typically positioned inside the cavity in order to retro reflect the pump beam. In the configuration presented in U.S. Pat. No. 5,406,409, the pump beam is retro reflected by a mirror (transparent to the signal and idler beams) placed inside the cavity. In the configuration presented in U.S. Pat. No. 5,781,571 the retro reflecting mirror is outside the cavity, but an additional turning mirror (transparent to the signal and idler beams) is placed inside the cavity. In U.S. Pat. No. 5,781,571 an OPO cavity incorporates a right angle prism; however, the prism reflects only the signal beam to form an SRO in a non-collinear configuration. The prism is not used to reflect either the pump or idler beams.
Cavity Length
The typical pump laser emits its energy in a very short pulse. The short duration of the pulse limits the interaction time between the parametric beams (signal and idler) and the pump beam. Therefore, it is crucial that the overall length of the cavity will be minimized in order to maximize this interaction time, and thus the conversion efficiency.
A device that is designed to operate as both DRO and DPO is the most efficient in converting pump energy to desired output at the signal and idler wavelengths. However, as stated above, it is impractical to fabricate multi-layer coatings that can satisfy the reflectivity and transmission demands across a broad spectral band including the pump, signal and idler wavelengths. Multiple sets of coated mirrors are required in prior art designs to cover a wide tuning range. Thus, the potential advantages of the DRO/DPO system is offset by the compromised performance of the resonator mirrors.
What is needed is a practical OPO with high conversion efficiency and with extended tuning range that can be manufactured from readily available optical materials at low cost.